Tignes' alpine lake is ice-encrusted, its frigid lid more than thick enough to support the weight of our group of divers. A hole has been chain-sawed into it, revealing a menacing eye of black water. I flop at its edge like an ungainly seal as our guide Alban slithers into its depths.

Passers-by on woolly hatted morning perambulations stop to goggle in disbelief at the unexpected sight. They probably think we're mad and, in a moment of panic, so do I. Then I roll with an inelegant plop into the ice hole. My chest feels tight, and shallow breathing fills my ears.

Common sense reminds me I'm in the French Alps in the middle of winter: more pine tree than palm tree, more tundra-like than tropical.

Still, for a moment my outraged mind expects coral, turtles and finding Nemo. But the only fish are green-grey trout that flit below, lips pursed in disapproval. Lanky weeds shiver amid a tumble of glacier-rounded rocks.

Then my guide Alban - who uses this lake to train Antarctic explorers in the niceties of ice diving - points upwards. In this topsy-turvy scuba-diving experience, it isn't only the temperatures that are Alice in Wonderland wrong. The best scenery is all above. I rotate, peer upwards - and find worry replaced by wonder.

On the surface of the lake the water, surrounded by a contrast of blinding snow, appeared jet-dark, almost solid. From below, it's as translucent as a mountain stream, with a pastel blueness so intense it reminds me of a curacao and vodka cocktail. Over ice, of course. Here, the ice is morphed into weird shapes and cracked into crazy-paving slabs where pockets of air wobble.

We follow Alban like Chinese kindergarten kids attached to a teacher's cord. The rope leads us from the ice hole we entered to our exit hole. We pass three other safety hatches cut in the ice, pierced with shafts of sunlight cast down by some cheerful, show-off God from a Bible illustration. Bubbles expelled by our regulators dance like pearls in the sunlight as they float upwards. They merge into ever-expanding air pockets that tremble under the ice like mercury, and reflect miniature images of passing scuba-divers.

The water is only a degree or two above freezing point. Despite our gear, we only have 20 minutes or so in this winter wonderland before risking hyperthermia. "Although we have never lost anyone yet," Alban chuckled during our preparation. "Only a number of toes."

We move on through an eerie, surreal world. I hear a hiss of oxygen, the uncanny creak of ice above, the thumping of my heart. Bubbles partner with sun and ice in a glittering waltz. I brush my gloved fingers along the iceshelf, in places needlepoint rough, in others smooth and lovely. When we finally break the surface, the world is harsh and bright, and my body walrus-heavy in the snow.